[llvm] r375403 - [MemCpyOpt] Fixing Incorrect Code Motion while Handling Aggregate Type Values

[Sam Elliott via llvm-commits]

Author: lenary
Date: Mon Oct 21 03:00:34 2019
New Revision: 375403

URL: http://llvm.org/viewvc/llvm-project?rev=375403&view=rev
Log:
[MemCpyOpt] Fixing Incorrect Code Motion while Handling Aggregate Type Values

Summary:
When MemCpyOpt is handling aggregate type values, if an instruction (let's call it P) between the targeting load (L) and store (S) clobbers the source pointer of L, it will try to hoist S before P. This process will also hoist S's data dependency instructions.

However, the current implementation has a bug that if one of S's dependency instructions is //also// a user of P, MemCpyOpt will not prevent it from being hoisted above P and cause a use-before-define error. For example, in the newly added test file (i.e. `aggregate-type-crash.ll`), it will try to hoist both `store %my_struct %1, %my_struct* %3` and its dependent, `%3 = bitcast i8* %2 to %my_struct*`, above `%2 = call i8* @my_malloc(%my_struct* %0)`. Creating the following BB:
```
entry:
  %1 = bitcast i8* %4 to %my_struct*
  %2 = bitcast %my_struct* %1 to i8*
  %3 = bitcast %my_struct* %0 to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %2, i8* align 4 %3, i64 8, i1 false)
  %4 = call i8* @my_malloc(%my_struct* %0)
  ret void
```
Where there is a use-before-define error between `%1` and `%4`.

Update: The compiler for the Pony Programming Language [also encounter the same bug](https://github.com/ponylang/ponyc/issues/3140)

Patch by Min-Yih Hsu (myhsu)

Reviewers: eugenis, pcc, dblaikie, dneilson, t.p.northover, lattner

Reviewed By: eugenis

Subscribers: lenary, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66060

Added:
    llvm/trunk/test/Transforms/MemCpyOpt/aggregate-type-crash.ll
Modified:
    llvm/trunk/lib/Transforms/Scalar/MemCpyOptimizer.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/MemCpyOptimizer.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/MemCpyOptimizer.cpp?rev=375403&r1=375402&r2=375403&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/MemCpyOptimizer.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/MemCpyOptimizer.cpp Mon Oct 21 03:00:34 2019
@@ -513,9 +513,13 @@ static bool moveUp(AliasAnalysis &AA, St
 
     ToLift.push_back(C);
     for (unsigned k = 0, e = C->getNumOperands(); k != e; ++k)
-      if (auto *A = dyn_cast<Instruction>(C->getOperand(k)))
-        if (A->getParent() == SI->getParent())
+      if (auto *A = dyn_cast<Instruction>(C->getOperand(k))) {
+        if (A->getParent() == SI->getParent()) {
+          // Cannot hoist user of P above P
+          if(A == P) return false;
           Args.insert(A);
+        }
+      }
   }
 
   // We made it, we need to lift

Added: llvm/trunk/test/Transforms/MemCpyOpt/aggregate-type-crash.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/MemCpyOpt/aggregate-type-crash.ll?rev=375403&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/MemCpyOpt/aggregate-type-crash.ll (added)
+++ llvm/trunk/test/Transforms/MemCpyOpt/aggregate-type-crash.ll Mon Oct 21 03:00:34 2019
@@ -0,0 +1,30 @@
+; RUN: opt -memcpyopt -S -o - < %s | FileCheck %s
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.14.0"
+
+%my_struct = type { i8, i32 }
+
+; Function Attrs: inaccessiblemem_or_argmemonly
+declare noalias i8* @my_malloc(%my_struct*) #0
+
+define void @my_func(%my_struct* %0) {
+entry:
+; CHECK: entry:
+  %1 = load %my_struct, %my_struct* %0
+  %2 = call i8* @my_malloc(%my_struct* %0)
+  %3 = bitcast i8* %2 to %my_struct*
+  store %my_struct %1, %my_struct* %3
+; CHECK-NOT: call void @llvm.memcpy.{{.*}}.{{.*}}.{{.*}}
+  ret void
+}
+
+attributes #0 = { inaccessiblemem_or_argmemonly }
+
+!llvm.module.flags = !{!0, !1, !2}
+!llvm.ident = !{!3}
+
+!0 = !{i32 2, !"SDK Version", [2 x i32] [i32 10, i32 14]}
+!1 = !{i32 1, !"wchar_size", i32 4}
+!2 = !{i32 7, !"PIC Level", i32 2}
+!3 = !{!"Apple LLVM version 10.0.1 (clang-1001.0.46.4)"}